The Mexican government on Thursday sent to the United States 29 top cartel operatives wanted by the American authorities, including one notorious drug lord whom U.S. officials had been seeking to bring to justice for 40 years, according to statements by the American and Mexican governments.

The handover included not only several powerful cartel leaders, but also some of the most prolific murderers in the annals of Mexican crime. The number and significance of the people sent to the United States at the same time made the event one of the most important efforts by Mexico in the modern history of the drug war to send traffickers to face charges in American federal courts.

The development came as the Trump administration was leaning hard on the Mexican government to step up its fight against the cartels, and Mexican officials’ concession appeared to be an early win for President Trump in what will likely be a longer struggle against the criminal groups.

Among those being flown to the United States was Rafael Caro Quintero, a founding member of the Sinaloa drug cartel, who was convicted in Mexico of masterminding the 1985 murder of Enrique Camarena, an agent with the Drug Enforcement Administration, Attorney General Pam Bondi said in a statement. Getting hold of Mr. Caro Quintero has for decades been all but an obsession among officials at the drug agency.

“As President Trump has made clear, cartels are terrorist groups, and this Department of Justice is devoted to destroying cartels and transnational gangs,” Ms. Bondi’s statement said. “We will prosecute these criminals to the fullest extent of the law in honor of the brave law enforcement agents who have dedicated their careers — and in some cases, given their lives — to protect innocent people from the scourge of violent cartels.”

The transfer of the wanted men, who had been in Mexican custody, came as a high-level delegation from Mexico arrived in Washington to meet with senior U.S. officials to hammer out a security agreement amid tension between the two nations. The Mexican Foreign Ministry issued a statement announcing the release of the cartel figures before Ms. Bondi issued her own statement.

“This action is part of the work of coordination, cooperation and bilateral reciprocity within the framework of respect for the sovereignty of both nations,” the Mexican statement said.

In recent weeks, the Trump administration has been engaged in a heated debate over how far — and how hard — to go in pushing the Mexican government to deal with the cartels, which have for years wreaked bloody violence in Mexico and smuggled untold amounts of illegal drugs into the United States.

Some White House officials have adopted an aggressive posture, advocating unilateral military action against drug lords and cartel infrastructure in Mexico to curtail narcotics, such as fentanyl, from crossing the border. Others have argued for a more pragmatic approach, saying that increased partnership with the Mexican government would ensure continued cooperation on issues like migration.

Amid these deliberations, Mr. Trump and his allies have placed enormous diplomatic and economic pressure on Mexico’s president, Claudia Sheinbaum, including by threatening to impose steep tariffs on her country.

On Thursday, at a joint news conference with Britain’s prime minister, Keir Starmer, Mr. Trump kept up that pressure, saying the flow of lethal drugs across the southern border had not stopped.

“The drugs continue to pour into our country, killing hundreds of thousands of people,” he said, even though U.S. overdose deaths have dropped recently, according to public health officials. “We’re losing substantially more than 100,000 people. I mean, dead.”

Still, Mexico’s decision to send the jailed traffickers to the United States was hailed in American law enforcement circles as a major victory and a clear signal that Ms. Sheinbaum planned to cooperate with the Trump administration in cracking down on the cartels.

“This is an unbelievably important moment and marks a true turning point,” said Ray Donovan, the former chief of operations for the D.E.A. “This shows President Sheinbaum’s willingness to work with us to target and dismantle the criminal organizations that have impacted the United States and Mexico for generations.”

Mr. Caro Quintero is an outsize figure in Mexico. He is also reviled by U.S. federal drug agents for the role he played in the torture and killing of Mr. Camarena, who was known as Kiki, while working undercover in Mexico. Mr. Camarena’s murder has long been viewed as a kind of catalyst that propelled American law enforcement deeper into Mexico’s cataclysmic war against the cartels.

After being sentenced to 40 years in prison, Mr. Caro Quintero was released from Mexican custody on a legal technicality in 2013 and returned to hiding in rural Sinaloa, his home state. He was ultimately captured by the Mexican authorities near San Simón, a town in Sinaloa, in 2022.

Only hours after he was detained, a military helicopter crashed outside the nearby city of Los Mochis, killing 14 Mexican marines onboard. Mexico’s president at the time, Andrés Manuel López Obrador, said then that the slain troops had been involved in the mission to capture the former crime lord.

Mr. Caro Quintero has been under indictment on several drug trafficking counts in Federal District Court in Brooklyn since 2020. And he could make an appearance there in front of a federal judge as early as Friday, one of the people familiar with the matter said.

Mexico was also releasing into U.S. custody Miguel Ángel Treviño Morales, an infamously brutal former leader of the Zetas cartel who was captured in Mexico in 2013, the people said.

Mr. Treviño, who is better known as Z-40, after his radio call sign in the Zetas, is widely viewed as one of Mexico’s most violent cartel operatives, having helped perfect the practice of using carnage as a message.

His organization was founded by highly trained and heavily armed Mexican commandos who were initially given the job of going after the gangs, but in the end sold their services to one in particular, the Gulf cartel. After riches — and intense bloodshed — followed, the Zetas, with Mr. Trevino in their upper ranks, went out on their own and eventually became one of Mexico’s most powerful and feared criminal organizations.

Mr. Treviño is facing overlapping drug charges in federal courts in Texas, including those in Austin and Laredo.

On the list of those sent to the United States were other leaders of — and killers for — the Zetas. There were also top operatives from other crime groups, including the New Generation Jalisco Cartel and La Familia Michoacana Cartel.

One of those taken into custody by American federal agents was José Ángel Canobbio Inzunza, said to be right-hand man of Iván Archivaldo Guzmán Salazar, a son of the notorious drug lord Joaquín Guzmán Loera, better known as El Chapo. Mr. Canobbio Inzunza, who is facing charges in Chicago, was arrested in Mexico only last week.